Explanation

A lower COF indicates a slipperier string.

The string-to-string coefficient of friction is the ratio of the friction force divided by the normal force pushing the strings together. The friction force opposes the main string's efforts to slide sideways back and forth across the cross strings during impact. The normal force is the force pushing the mains and crosses together, and it is a combination of string tension and impact force. Strings with a low string-to-string COF will slide more readily and increase ball spin during the snap-back phase. The string-to-string COF is only a measure of a string sliding on itself. Separate measurements would need to be done for each string sliding on another model of string. That is because materials slide differently on each other than they do on themselves. Of the two friction COFs that we measure (string and ball), the string-to-string COF is probably the most important.